So called "fly-by-wire" aircraft are typically extremely sophisticated and powered by jet engines. In such aircraft, there is no direct mechanical connection between the controls operated by the pilot and the control surfaces on the aircraft itself. Rather, electrical and/or hydraulic circuits interconnect the controls and the control surfaces. Thus, in order for control of the control surfaces to be effective, it is necessary that there be some source of power, such as an electric generator or a hydraulic pump or both. In the usual case, such components are driven by the main propulsion engine or engines of the aircraft and in normal operation, no difficulties are encountered.
However, there is always the possibility that the power source will be disabled as a result of flame out of the main propulsion engine or engines and when that occurs, control over the aircraft is lost since there is no power to alter control surfaces in response to pilot commands.
The difficulty is accentuated at high altitude because of the difficulty of restarting turbine engines at such altitudes.
It has, therefore, been proposed to provide an emergency power unit for such aircraft. The emergency power unit frequently is a small gas turbine engine employed to drive an electric generator or a hydraulic pump or both in an emergency situation. In the usual case, it is necessary that such an emergency power unit be capable of being brought from a fully quiescent state to full operation in two-three seconds. And while starting of such an emergency power unit at low altitude does not pose much of a difficulty, at high altitudes, it is just as susceptible to difficult starting as the main propulsion engines of a typical aircraft.
In order to provide for rapid starting at altitudes, various systems have been proposed In one, hydrazine is caused to decompose and the resulting gases directed against a turbine wheel with an auxiliary nozzle. The turbine wheel is thus accelerated and caused to drive a compressor, which in turn provides compressed air to a conventional combustor so as to allow combustion of fuel therein to provide the gases of combustion to drive the turbine wheel through a main nozzle. This system has not found a great deal of favor because of the nature of hydrazine.
In other systems, a compressed oxidant is stored in a storage vessel and the gas turbine engine of the emergency power unit provided with an axillary combustor The stored oxidant is utilized to oxidize fuel within the axillary combustor and the resulting gases of combustion are employed to accelerate the turbine wheel. The turbine wheel in turn drives a compressor and when self-sustaining speed is reached, the same operates normally as a gas turbine engine and use of the axillary combustor may be terminated.
Because of the need to render the emergency power unit operable within just a few seconds, substantial internal temperature rises are caused to occur within a very short period of time, requiring that considerable effort be taken to accommodate thermal expansion of the various components. If this is not done properly, extremely flammable gases and oxidant flowing through expansion joints in the construction may come in contact with each other at undesired locations and cause a rapid burnout of system components.
The present invention is directed to overcoming one or more of the above problems.